Plansifters are used for the separation and grading of cereals and all products resulting from their breaking and milling. Plansifters generally are formed of channels joined to and driven by a central body containing a rotating counterweight. Plansifters are suspended by means of rods or other elastic devices so they can freely move within a circular or elliptical-like path. Separation within each plansifter is managed by designing a sieve stack, which is a stack composed of multiple sieves. Depending on, for example, the granularity of the grading or other factors, a stack can have various numbers of sieves. While there is no limit to the number of sieves in a stack, some models of sifters can have less than 10 sieves, and other models of sifters can have 30 or more sieves. However, commonly, sifters have less than 30 sieves, such as about 24 to 27 sieves.
Each sieve includes a sieve cloth attached (e.g., glued, stapled, etc.) onto a removable frame that sits within a sieve box. The sieve cloth receives the material to be sieved from the top and, aided by mechanical movement of the stack, allows particles smaller than the mesh opening of the sieve cloth (generally from 112 μm to 5000 μm) to fall into a sieve box below the sieve cloth. Particles larger than the mesh opening continue to move on the sieve cloth until reaching a dedicated falling zone in the sieve box. The removable frame includes dividers that separate the sieve cloth “cleaning zones.” Typically, there are about six to nine cleaning zones for each sieve. The sieve box serves as the outer boundary of the sieve and redirects the product falling from the cloth onto sieves below. Depending on the type of sieve, explained in greater detail below, each sieve also includes multiple cleaners/expellers (typically one per cleaning zone) that freely move based on the mechanical movement of the stack and aid the movement of the particles through the mesh openings within the sieve cloth or to the falling zone. There currently are two types of sieves: backwire sieves and combined sieves, which are described below in relation to FIGS. 1 and 2.
FIG. 1 illustrates a backwire sieve 100. The backwire sieve 100 includes the sieve box 102. The sieve box 102 includes a sieving zone 102a and a falling zone 102b. The sieving zone 102a includes a sieve cloth (not shown) that provides the sieving action. The sieve cloth allows particles of a certain size or smaller to fall through. Once the particles fall through the sieve cloth, they are expelled out of the sieve box 102 through side openings 102c and fall down a channel to either another sieve with the sieve stack or a collection point. The side openings 102c can be on one side of the sieve box 102 or can be on two or three sides of the sieve box 102, depending on the desired configuration. The larger particles that remain on the sieve cloth reach the falling zone 102b caused by the mechanical movement of the sieve 100. At the falling zone 102b, the larger particles fall through a separate channel to either another sieve below within the sieve stack or a collection point.
The backwire sieve 100 also includes the removable frame 104 that sits on the sieve box 102, a backwire grille 106 that sits within the sieve box 102 below the frame 104, and a bottom sheet 108 that sits within the sieve box 102 and below the backwire grille 106 when installed into a plansifter. The backwire grille 106 can be fastened to the frame 104 or lie loose between frame 104 and the sieve box 102. The sieve box 102 and the frame 104 can include dividers 110a and 110b that divide the sieve 100 into multiple, separate expulsion zones 112a (e.g., dividers 110a) and cleaning zones 112b (e.g., dividers 110b). Specifically, the dividers 110a on the sieve box 102 divide the sieving zone 102a into multiple different expulsion zones 112a. In some embodiments, there can be the same number of expulsion zones 112a as side openings 102c, such that each expulsion zone 112a corresponds to a separate side opening 102c. The dividers 110b on the frame 104 divide the sieving zone 102a into multiple different cleaning zones 112b, above the expulsion zones 112a. The combination of an expulsion zone 112a and a cleaning zone 112b spans from the bottom sheet 108 to the sieve cloth (not shown) attached to the top of the frame 104.
Within each cleaning zone 112b above the backwire grille 106 and below the sieve cloth is an untethered cleaner (not shown). During operation of the sieve 100, the cleaner has an erratic bouncing movement so that it continuously taps the sieve cloth above and avoids the product from choking the mesh openings in the sieve cloth. The distance between the sieve cloth and the backwire grille 106, therefore, should be a set distance so that that cleaner can contact the sieve cloth.
Within each expulsion zone 112a below the backwire grille 106 is an expeller (not shown). The expeller pushes the particles that fall through the sieve cloth to the side openings 102c of the sieve box 102. Because the expeller does not need to contact the backwire grille 106, there is no distance requirement between the bottom sheet 108 and the backwire grille 106.
FIG. 2 illustrates a combined sieve 200. The combined sieve 200 is substantially similar to the backwire sieve 100, except that the combined sieve 200 lacks the backwire grille 106. Thus, elements of the combined sieve 200 that are similar to the elements described above for the backwire sieve 100 are similarly numbered. The word “combined” in the term “combined sieve” connotes that one device is present that carries out both the cleaning and ejecting or expelling functions of the two devices referred to as a cleaner and an expeller described above.
The combined sieve 200 includes the sieve box 202 with the sieving zone 202a and the falling zone 202b. The sieve box 202 further includes side openings 202c that allow particles that fall by gravity through the sieve cloth (not shown) to escape the sides of the sieve box 202. The side openings 202c can be on one side of the sieve box 202 or can be on two or three sides of the sieve box 202, depending on the desired configuration. The falling zone 202b allows the particles too large to fall through the sieve cloth to instead move to the side of the sieve 200 and fall to a sieve below.
The combined sieve 200 also includes the removable frame 204 that sits on the sieve box 202 and a bottom sheet 208 that sits within the sieve box 202 and below the sieve cloth. The sieve box 202 and the frame 204 can include dividers 210a and 210b that divide the sieve box 202 and the frame 204 into separate expulsion zones 212a and cleaning zones 212b, respectively. Thus, like the expulsion zones 112a and cleaning zones 112b above, the combination of an expulsion zone 212a and a cleaning zone 212b spans from the bottom sheet 208 to the sieve cloth (not shown) attached to the top of the frame 204.
Because the combined sieve 200 lacks a backwire grille, within each cleaning zone 212b can be a combined cleaner 214 that performs both the cleaning and expelling described above. As the combined sieve 200 moves, the combined cleaner 214 moves around and bounces about the cleaning zone 212b erratically, tapping the sieve cloth above and pushing fine particles out of the sieve box 202 through the side openings 202c. The distance between the sieve cloth and the bottom sheet 208 should be a fixed distance so that that combined cleaner 214 can contact the sieve cloth with enough force to clean the sieve cloth.
With the above configurations of the backwire sieve 100 and combined sieve 200 in mind, one of the main constraints of plansifters is stack height. Stack height is important because, during the work phase, the stack is compressed between covers so that the product placed between a sieve and the underneath cannot escape from the stack. If the stack height is less than a certain value, compressive sealing is not guaranteed. If stack height is more than a certain value, there are problems inserting the complete stack into a channel. Further, there must be enough volume for the incoming product to avoid choking and clumping. This problem also becomes more of a concern as the fine percentage rises and increases the sieve cloth throughput occupying the finite underlying volume. The lesser sieve height, the lesser the volume underneath the sieve and/or available to throughput. For this reason, channels using combined sieves generally contain 1 or 2 sieves more than channels using backwire sieves, considering the same product flow rate.
There are situations where backwire sieves or combined sieves can be employed, and situations where one may be considered better than the other. However, because of the simplicity in managing sifting stack schemes, maintenance of sieves, spare parts, and the like, there are no stacks comprising both backwire sieves and combined sieves. Also, because of different distances from the bottom sheet to the sieve cloth between backwire and combined sieves, it is not possible to switch from one style to the other, such as by removing the backwire grille.
Further, drawbacks exist for the combined cleaner of combined sieves as compared to the separate cleaners and expellers in backwire sieves. For example, the uneven surface of the backwire grille provides for more erratic movement of the cleaners in backwire sieves as compared to the even bottom sheet with combined cleaners in combined sieves. Further, despite the erratic movement of combined cleaners caused by the mechanical movement of the sieve stack, the combined cleaners still move over preferential paths that results in better cleaning in certain areas versus others.
Accordingly, aspects of the present disclosure solve the above issues associated with the incompatibility between backwire and combined sieves by providing a single sieve that can be switched between backwire and combined configurations. Further, aspects of the present disclosure solve the above issues associated with combined cleaners by providing a dynamic center of gravity.